Heating and cooling degree days are terms you may run across. To determine a degree day first a reference
temperature needs to be picked. 65 F is often the chosen value. The degree day will often give the
reference value such as DD-65.

A heating degree day using DD-65 is the number of degrees the mean temperature is below 65 F. The mean
temperature can be determined by averaging the high or low for the day, averaging the hourly temperatures
for the day or averaging the temperature in even finer intervals. Make sure to determine how the
mean is determined. Here is an example of finding the DD-65 value. Suppose the mean temperature for a
day is 45. The number of heating degree days is 65 - 45 = 20 heating degree days. Any mean temperature
of 65 F or above yields 0 heating degree days.

A cooling degree day using DD-65 is the number of degrees the mean temperature is above 65 F. Here is an
example of finding the DD-65 value. Suppose the mean temperature for a
day is 75. The number of cooling degree days is 75 - 65 = 10 cooling degree days. Any mean temperature
of 65 F or below yields 0 cooling degree days.